PLMN search is an important feature for a mobile device in a mobile communication system since it directly impacts the user experience when a user powers up the mobile device to find a network or has to do a network scan. Therefore, the faster and more accurate a PLMN search can finish, the better will be the user experience.
Normal PLMN search consists of two consecutive steps, frequency scan and cell scan. Frequency scan is carried out first for all candidate frequencies, for example in an LTE system for E-UTRA Absolute Radio Frequency Channel Numbers (EARFCNs), to identify frequencies, which most likely contain a cell. After a complete frequency scan, a cell scan is carried out for each detected frequency, in an LTE system for each detected EARFCN, to check whether there are cells there or not.
There are two drawbacks with the above PLMN search method. First of all, the time spent on the frequency scan can be quite long when the number of candidate frequencies is large. For example, in an LTE system totally 692 EARFCNs are specified. Secondly, the system parameters used during frequency scan are normally biased towards a worst case scenario at sacrifice of some performance metrics. For example, with respect to a frequency offset, in the absence of any a priori knowledge of the frequency offset as is the case when the mobile device powers up, a worst case scenario would mean a large frequency offset is assumed to exist throughout the entire frequency scan stage. Therefore, the frequency scan method has to be robust against large frequency offsets, potentially at the cost of sensitivity.